Efficient operation of concrete precasting and prestressing manufacturing plants requires that the molds be reused as quickly as possible. In this regard, steam curing of the portland cement concrete systems is generally used as it has been found that accelerated curing of non-latex modified compositions with dry heat results in significant cracking which makes such materials structurally useless (See FIG. 1).
During the steam curing operation the concrete mix is left to "dwell" without heat, for a period of from about 2 to 5 hours. Heat is then applied and a temperature rise of 30.degree. F to 50.degree. F per hour is used. Such curing method requires at least from about 12 to 18 hours cure time and even with such elaborate precautions the concrete mixes never attain the strengths which are obtained by use of unaccelerated cures. Further, in the case of latex-modified cement products, the use of steam has been found to be detrimental to obtainment of optimum strength in such products.
The effect of temperature during cure of conventional non-latex modified portland cement concrete systems is illustrated on page 84 of the January, 1963, issue of The Journal of American Concrete Institute and pages 517-3 thru 517-9 of the ACI Manual of Concrete Practice, Part 3, 1968. The data set forth by the referred to publications clearly show that utilization of a temperature greater than about 70.degree. F, in either a steam cure or dry cure, results in a weakening of the concrete mix (based on a standard 28 day normal cure strength).
It is the primary object of the present invention to provide a means of achieving a non-distressing accelerated cure of precast or prestressed portland cement concrete systems to obtain optimum strength in the shortest possible time.